Most commercially available powered vehicle seat lifts provide for both vertical lifting and back and forth tilting of the seat cushion, as well as back and forth, lateral adjustment. These are typically referred to as six way adjusters, the six ways being up and down, back and forth tilt, and back and forth lateral adjustment. An example may be seen in co assigned U.S. Pat. No. 4,664,351 to Borlinghaus. Powered jack screw units beneath the front and back of the seat cushion frame are rigidly attached at their lower ends to a floor plate, and attached at their upper ends to the seat cushion frame through a so called lost motion pivot, consisting of a pin that moves in a substantially horizontal slot. The pins and slots allow vertical lift to be applied to the seat cushion frame as well as allowing the ends of the frame to swing in an arc having a horizontal component. The same allowance for compound motion can be provided through the use of an additional transfer link or links between the end of the jack screw and the seat cushion frame. Less common, though known, are parallel acting seat lift mechanisms, which remain substantially parallel to their previous positions as they are raised or lowered, but lack the ability to tilt back and forth independently. An example may be seen in co assigned U.S. Pat. No. 2,919,744 to Tanaka. Even with a parallel lift, allowance must be made for some horizontal motion concurrent with the vertical, through pin and slot connections or extra links. An inherent problem with any lift mechanism that uses pin and slot connections or extra links, especially when located beneath the front of the seat cushion frame, is that a rocking motion by the seat occupant can cause a perceptible up and down motion of the seat cushion frame, sometimes referred to as seat chuck. While the basic operation of the seat is not affected, a perceptible motion of the seat is thought to be undesirable in terms of quality, or at least the perception of quality. However, there was previously no known means of providing for the necessary compound motion of the seat frame without the kinds of lost motion connections that were inherently loose.
Recently, a seat lift design disclosed in co assigned U.S. Pat. No. 5,586,740 has provided a six way seat adjuster without the need for the lost motion connections. Powered jack screw units beneath the front and rear of the seat cushion frame have close fitting pivots at both the upper end, where they attach to the seat cushion frame, and the lower end, where they attach to the floor pan. In order to provide stability, a brace located between the two jack screw units is closely pivoted to both the floor pan and the seat cushion frame, running diagonally. Having only close fitting pivots, and no pin and slot connections or extra links, the seat cushion frame is able to move up, down, or tilt back and forth with essentially no perceptible play or chuck. A variation of the design is disclosed in co pending and co assigned application Ser. No. 08/548,892, filed Oct. 26, 1995. This design replaces the rear jack screw unit, which is essentially vertical, with a nearly horizontal jack screw unit, the moving end of which is pivoted to the seat cushion frame through an intermediate, single bell crank that is also centrally pivoted to the floor pan. The single bell crank acts both as a force transferring connection to the seat cushion frame, and as a stabilizing brace. As with the design in U.S. Pat. No. 5,586,740, the absence of pin-slot connections or multiple link connections provides a solid occupant feel with substantially imperceptible response of the seat cushion frame to a rocking motion. Both designs, however, require two jack screw units, and their attendant motors, drive shafts and transmissions, which are costly.